Status and outlook on superconducting fault current limiter development in Europe

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1 Status and outlook on superconducting fault current limiter development in Europe Mathias Noe 1, Christian Schacherer 1 1. Institute for Technical Physics, Forschungszentrum Karlsruhe, Germany Abstract: The application of superconducting fault current limiters (SCFCLs) in power systems is very attractive because SCFCLs offer superior technical performance in comparison to conventional devices to limit fault currents. Negligible impedance at normal conditions, fast and effective current limitation within the first current rise and repetitive operation with fast and automatic recovery are the main attributes for SCFCLs. In recent years there has been worldwide a significant progress in R&D of SCFCLs. Especially in Europe several projects demonstrated the feasibility of SCFCLs in medium voltage applications. This paper gives an extended review of the present state-of-the-art of SCFCL development, with favourable applications from a European point of view. It is further discussed which SCFCL concepts and materials seem most promising for future applications. It can be summarized that SCFCLs are, at present, not commercially available but several successful field tests demonstrated the technical feasibility of SCFCLs at the medium voltage level. The development of a transmission type SCFCLs has been started in Germany and the US and first successful intermediate tests are expected soon. In general, considerable economical and technical benefits can be achieved by applying SCFCLs at the distribution and transmission voltage level. Keywords: Fault Current Limiters, Short-circuit 1. INTRODUCTION The short-circuit current level in electric power systems will increase in future due to increased de-centralized generation and more urban power systems with high power density. One favorable device to limit short-circuit currents without increasing the power system impedance at normal operation is the superconducting fault current limiter (SCFCL). Due to its fast and effective short-circuit current limitation within the first current rise, the automatic recovery and the fail safe operation, the application of SCFCLs is very attractive. The general operation modes and some important parameters of SCFCLs are shown in Fig. 1. During normal operation the SCFCL impedance is negligible and the SCFCL is designed to carry the rated current i r. Without SCFCL the short-circuit current would rise up to the peak value i p, but due to the fast limitation the current is limited to a maximum current i max within the first current rise. Current Normal operation Short-circuit Recovery î p Without SCFCL Current: Rated current i r, î r Max. limited current î max Peak short-circuit current î p Time: î max Fault clearing time t d Recovery time t r Others: î r Limitation factor î max /î r t d Time Figure 1: Operation modes and important parameters of SCFCLs t r After the clearing of the short-circuit usually, a short recovery time t r is needed to re-cool the SCFCL to its original temperature. The recovery time depends on the SCFCL type and the design and ranges from zero up to less than a minute. An important parameter for SCFCLs is the so-called limitation factor, the ratio of the maximum limited current i max divided by the peak value of the rated current î r. This paper shortly describes the most attractive SCFCL concepts, presents favorable SCFCL applications from a European point of view and shows the state-of-the-art of SCFCL development. Due to the fast progress with SCFCL projects using YBCO coated conductors a comparison of the quench behavior of different YBCO coated conductors is added. 2. FAVORABLE SCFCL CONCEPTS In general, many different SCFCL concepts are known. The most popular ones are the resistive type SCFCL, the bridge type SCFCL, the DC biased iron core type SCFCL and the shielded core type SCFCL. A comprehensive summary on different SCFCL concepts is given in [1]. At present, the author knows no new active project to further develop a shielded core type SCFCL. Therefore, this paper shortly describes the function of resistive type SCFCLs, bridge type SCFCLs and DC biased iron core type SCFCLs Resistive SCFCL The electrical circuit of a resistive type SCFCL is shown in Fig. 2. During normal operation the superconducting material is in the superconducting state and the resistance R SC is zero. At the short-circuit the current rises above the critical current I c of the superconductor and Corresponding author: M. Noe, mathias.noe@itp.fzk.de 529

2 curve resulting in a significant increase of the apparent coil inductance. The superconductor is used only for the DC windings, which means that this type of SCFCL needs the smallest amount of superconductor and that no superconductor AC losses have to be considered. The major characteristic of a DC biased iron core type SCFCL are: + no superconductor quench, immediate recovery + superconductor for DC only + adjustable trigger current - high volume and weight - high impedance during normal operation Fig. 2: Electrical circuit of a resistive SCFCL with parallel resistance thus a resistance R SC develops rapidly. This resistance heats the superconductor usually above its critical temperature T c and this means that after the short-circuit time t d (equal to fault clearing time) a recovery time is needed to re-cool the superconductor to its operation temperature. A resistance in parallel R SH is needed to avoid hot spots during quench, to adjust the limiting current and to avoid overvoltages due to the fast current limitation. In comparison to other SCFCL types the resistive type SCFCL is the most compact type and offers full recovery within a few seconds. The major characteristic of a resistive SCFCL are: + compact + low weight + fail safe - current leads to low temperatures 2.2. DC biased iron core SCFCL It is well known that the non-linear characteristic of iron cores can be used for current limiting device. The basic electric scheme of a DC biased iron core SCFCL is shown in Fig. 3. At normal operation currents both iron cores are driven into saturation with direct currents i DC1 and i DC2. As a result the impedance is low, consisting mainly of the two air coil impedances L 1 and L 2 and their resistances. In the case of a fault, the large AC current i AC will alternately drive the two coils out of saturation and into the region of high permeability on the magnetization Fig. 3: Electrical circuit of a DC biased iron core type SCFCL 2.3. Bridge type SCFCL The basic electrical circuit of this bridge type SCFCL is shown in Fig. 4. A full rectifier bridge, a limiting coil and a voltage source is needed for this type of SCFCL. At normal operation the amplitude of i AC is lower than the DC current I 0 and therefore all diodes are conducting. The limiting coil L sees the DC current only. If i AC gets higher than I 0 then the diodes D 3 and D 4 will arrest at the positive half cycle and thus the current is limited by the inductance L. At the negative half cycle the diodes D 1 and D 2 will arrest if i AC is larger than I 0. Thyristors instead of diodes enable a fast turn-off of the current within a half cycle. The major characteristic of a bridge type SCFCL are: + no superconductor quench (L can be non-sc) + immediate recovery + adjustable trigger current (depends on I 0 ) - not fail safe - losses in semiconductors Fig. 4: Electrical circuit of the basic circuit of a bridge type SCFCL 3. SCFCL APPLICATIONS It is obvious that there are many potential applications in power systems for SCFCLs. An extended overview about the different applications is given in [1]. The applications can be divided into applications at distribution and transmission voltage level and the technical and economical benefits depend very much on the specific application and the specific situation in the power system. This has been described earlier in [2]. Favorable SCFCL applications at distribution voltage level are: - SCFCL in busbar coupling - SCFCL in transformer feeder - Coupling of dispersed generation with SCFCLs 530

3 - SCFCL in generator feeder location - SCFCLs in power plant auxiliaries - SCFCL in ship propulsion systems Favorable SCFCL applications at transmission voltage level are: - Subgrid coupling with SCFCLs - SCFCLs in busbar coupling - SCFCLs to avoid sequential tripping The European power system is presently characterized by a relatively small increase in load, an increasing connection of renewable energy (wind, solar) and an increasing cost pressure due to market liberalization. In general, this situation is not favorable for implementing new current limiting devices. Therefore, the most attractive SCFCL applications from a European perspective are those with considerable savings. At present, SCFCLs in power plant auxiliaries [3] and subgrid coupling with SCFCLs [4] are very promising because in each application a few Mio. can be saved by applying SCFCLs. In future, the application of current limiting systems, e.g. high power feeders with a combination of superconducting cables and current limiters seem very attractive or a current limiting transformer seems quite favorable, because considerable additional benefits can be achieved in comparison to a stand alone SCFCL application. 4. STATE-OF-THE-ART OF SCFCL R&D At present, SCFCLs are not commercially available but many projects worldwide are on their way to develop prototype SCFCLs for field tests. An overview about the most recent and actual major SCFCL projects is given in Table 1. It can be seen that most of the projects follow the resistive type SCFCL and that recently more and more projects use YBCO coated conductors as superconductor. The latter is mainly due to the progress in R&D of YBCO coated conductors and the availability of this material in adequate lengths since Distribution level SCFCLs Up to now, three successful field tests of distribution level SCFCLs took place. The first one was performed with a shielded core type SCFCL by ABB (Asea Brown Bovery) in 1996 in Switzerland [4]. This test took one year and was successful but without limiting a short-circuit at that time. After this test ABB did not longer follow this type of SCFCL because of the relative large size of this concept. At present, no large activity is known to the authors, following the shielded core type SCFCL. A second field test with a distribution type SCFCL was performed with a 10 MVA, 10 kv resistive SCFCL using bulk MCP-BSCCO 2212 superconductor [5]. This test took place from March 2004 until April 2005 in a busbar coupling location in the grid of RWE in Netphen, Germany. The test results showed that the safety measures and the concept to integrate SCFCLs into the grid work properly and underlined that this type of medium voltage SCFCL is technically feasible. The high temperature superconducting components in the geometry of bifilar coils made out of MCP-BSCCO 2212 tubes showed no degradation during the field test. Unfortunately, no three phase fault occurred during the time of the field test. Figure 5 shows the HTS insert consisting of 90 bifilar coils, 30 in series for each phase. A third successful field test started in China in 2005 with a 6 kv, 1.5 ka diode bridge type SCFCL [6]. This prototype was the first ever to experience a three phase fault in the grid and the field tests included several insulation tests and short-circuit tests. This SCFCL was located in a substation in Hunan, China. In comparison to the basic electric circuit of a diode bridge type SCFCL this type used several IGCTs with a shunt resistance in series to the superconducting limiting coil. Recently, two successful short-circuit tests of resistive type medium voltage SCFCLs using YBCO coated conductor material were reported [7]. In Korea a successful high power laboratory short-circuit test with a single phase 13.2 kv, 630 A SCFCL demonstrator was performed in In March 2007 another successful test with a single phase 7.5 kv, 267 A SCFCL demonstrator was reported in Germany. It can be summarized that medium voltage SCFCLs of each type are technically feasible and that it is very likely to see soon first pre-commercial SCFCL application for this voltage level. Further R&D effort should concentrate on reducing cost and demonstrating long term stability and reliability Transmission level SCFCLs A major reason to develop SCFCLs for the transmission voltage level is that there exists no conventional device with an active current limitation and low impedance during normal conditions. In addition, studies showed that high savings and many benefits can be achieved at this voltage level. The first project to develop a HTS transmission type SCFCL started in 2003 in the US [8]. During the project it turned out that the concept of a so-called matrix type fault current limiter (MFCL) is difficult to adapt to high voltage levels. As a result this project was discontinued for some time. In 2006, new partners joined this project and the concept was changed to a resistive SCFCL with YBCO coated conductors. A successful intermediate test was reported in [9] and the main objective is to develop a 138 kv, three phase resistive SCFCL until In 2005 a project in Germany started to develop a 110 kv, 1.8 ka single phase resistive SCFCL until This project uses MCP-BSCCO 2212 tubes with a magnetic field assisted quench concept [10]. The status and the concept are reported in [11]. A major challenge in this project is to develop the HTS component for all relevant short-circuit levels. In June 2007 the US Department of Energy announced two new transmission level SCFCL projects [12]. A 115 kv three phase resistive type SCFCL and a 138 kv three phase DC biased iron core type SCFCL will be developed. 531

4 5.1. Samples and test circuit Low stabilized CC with a silver cap-layer only and high stabilised CC with silver layer and additional copper or stainless steel coating were tested. Table 2 lists the specifications of the test samples. All samples were 11cm long and the voltage along the conductor was measured each single centimeter. A transformer supplied the 50 Hz sinusoidal current. The peak value of the prospective current (i p ) was adjusted with an infinitely variable resistance in serial to the test samples. Therefore, low short circuit currents (<3 times I c ) as well as high short circuit currents with more then 10 times I c can be adjusted. For the comparison of the CC with their different critical currents i p was normalized with I c. Fig. 5: HTS insert of 10 MVA, 10 kv resistive SCFCL using MCP-BSCCO 2212 bifilar coils (CURL10) Within the last phase of the Korean DAPAS program it is foreseen to develop a 154 kv SCFCL until It is not yet decided which type of SCFCL will be taken. Intermediate successful tests with 22.9 kv demonstrators were performed with a hybrid type and a resistive type SCFCL. A major challenge for transmission level SCFCLs will be to demonstrate a reliable high voltage insulation concept for AC voltages and lightning impulses Quench behaviour at low short-circuit currents The high stabilised coated conductors are unable to limit the short circuit current noticeable due to their low resistance (see Fig. 6). There is no significant increasing of the resistance of the high stabilised CC even after 4 net cycles. The low stabilised coated conductors limit the short circuit current on the other hand already at low short circuit currents evidently with a limitation factor (f lim ) around QUENCH BEHAVIOUR OF YBCO COATED CONDUCTORS The recent progress in R&D of YBCO coated conductors (CC) make them attractive candidates for resistive SCFCLs. Therefore, a comparison of industrial available coated conductors has been performed. Coated conductors are able to handle currents over I c the so called quench for some net cycles. The behavior at these over currents depends strongly on the homogeneity of the critical current along the coated conductor, its stabilization and the amplitude of the over current as well. Table 2. Specification of the tested coated conductors Nr. Stabilisation Homogeneity Width (mm) 1 Silver (rd. 2.5µm) 1% rd Silver and copper 5% rd. 4 3 Silver and stainless steel >14% rd. 4 4 Silver (rd. 0.5µm) >9% rd Silver (rd. 5µm) >10% rd. 10 Fig. 6: Comparison of current and voltage at low level short circuit currents (sample length 11cm, T=77K) This short circuit level is the major challenge for inhomogeneous coated conductors. The slow increasing current in the beginning of low short circuit causes unbalanced resistance along the conductor with a low stabilisation. Fig. 7 shows the voltage and the resistance distribution respectively of a low stabilised and inhomogeneous coated conductor at low short circuit currents. Sections with higher critical currents may not able to build up a sufficient resistance to protect sections with lower critical currents of overheating. Therefore, the voltage drops only on a few sections. This may causes a damage of the coated conductor. With a higher stabilization, the unbalances oft the resistivity gets efficiently smoothed. Fig. 8 shows the more balanced voltage distribution on an inhomogeneous but high stabilised coated conductor. 532

5 very high copper stabilised one shows a more or less noticeable limitation. The low stabilised coated conductors reach limitation factors around 4 at this level. The inhomogeneous coated conductors show an uncritically distribution of the resistance at this short circuit level. Even sections with lower critical currents build up a sufficient resistance to protect sections with higher critical currents. Fig. 7: Voltage distribution of a low stabilised and inhomogeneous coated conductor measured each centimeter at low short circuit current Fig. 8: Voltage distribution of a high stabilised and inhomogeneous coated conductor measured each centimeter at low short circuit current 5.4. Summary of quench behaviour All of the tested coated conductors showed the expected quench behaviour in respect to their stabilisation and critical current homogeneity. Very high stabilised coated conductors can hardly limit the current at all short circuit current levels. On the other hand high stabilised coated conductors are able to limit the short circuit current at the high level. The low stabilised coated conductors reach the highest limitation factors at all and limit the current at every short circuit current level. The following statements summarize the results of the made study: Inhomogeneous coated conductors with low stabilisations are hardly able to withstand low short circuit currents. They need a higher stabilisation for the protection of hot spots. With the increasing of short circuit current amplitudes the importance of the critical current homogeneity diminishes. A good stabilisation can smooth the inhomogeneity of the coated conductor even at low short circuit currents to the disadvantage of limitation ability. Homogeneous coated conductors are able to limit a wide range of short circuit current amplitudes without the risk of damage. 6. SUMMARY AND OUTLOOK SCFCLs are new and attractive devices in power systems to limit short-circuit currents in power systems. At present SCFCLs are not commercially available but many projects underlined their feasibility in medium voltage level applications. In addition, several projects are on their way to develop SCFCLs for the transmission voltage level. Most present projects follow the resistive type SCFCL, very likely because of the simple concept and the low volume and weight. In Europe, the most favourable SCFCL applications are those with high savings. This is especially the case in power system auxiliaries and in the coupling location of 110 kv subgrids. The investigation of YBCO coated conductors shows that low level short-circuits has to be taken into account for the tests of SCFCLs and that a good stabilization of the conductor is more important for quench stability than the I c homogeneity. Fig. 9: Comparison of current and voltage at high level short circuit currents (sample length 11cm, 77K) 5.3. Quench behaviour at high short-circuit currents All of the tested samples show a limitation of the current at high short circuit currents (see Fig. 9). Even the REFERENCES 1. Noe M, Steurer M, Supercond. Sci. Technol. 20 (2007) R15-R Noe M, Oswald B R 1999 IEEE Trans. Appl. Supercond Pfeiffer K, Schwarz H 2006 Applications of resistive high 533

6 temperature superconducting fault current limiters in power-station service plants submitted to 6 WSEAS International Conference on Power Systems Sept Lisboa Portugal 4. Neumann C 2006 Superconducting fault current limiter (SFCL) in the medium and high voltage grid IEEE PES Meeting August 2006, Montreal 5. Paul W, Lakner M, Rhyner J, Unternährer P, Baumann T, Chen M, Widenhorn L, Guérig A 1997 Inst. Phys. Conf. Ser Kreutz R, Bock J, Breuer F, Juengst K-P, Kleimaier M, Klein H-U, Krischel D, Noe M, Steingass R, Weck K-H 2005 IEEE Trans. Appl. Supercond. 15 No Superconductor week 2007, Vol. 21 Nr Hui D, Wang Z K, Zhang J Y, Zhang D, Dai S T, Zhao C H, Zhu Z Q, Li H D, Zhang Z F, Guan Y, Xiao L Y, Lin L Z, Li L F, Gong L H, Xu X D, Lu J Z, Fang Z, Zhang H X, Zeng J P, Li G P, Zhou S Z 2006 IEEE Trans. Appl. Supercond. 16 No Superconductor Week 2003 Intermagnetics receives $6 million award for FCL program Superconductor Week 17 No Elschner S, Breuer F, Walter H, Bock J 2005 Magnetic field assisted quench propagation as a new concept for resistive current limiting devices Proceedings EUCAS Conference 2005 Vienna, Noe M, Kudymow A, Fink S, Elschner S, Breuer F, Bock J, Walter H, Kleimaier M, Weck K-H, Neumann C, Merschel F, Heyder B, Schwing U, Frohne C, Schippl K, Stemmle M 2007 Conceptual design of a 110 kv resistive superconducting fault current limiter using MCP-BSCCO 2212 bulk material, presented at ASC2006, Seattle and to be published at IEEE Transactions on Applied Superconductivity DOE press release 27 June 2007, DOE Provides up to $51.8 Million to Modernize the U.S. Electric Grid System Table 1. Important FCL projects (Status July 2007) Lead Company Country / Year 1) Type Data 2) Phase Superconductor ACCEL/Nexans SC Germany / 2004 Resistive 6.9 kv, 600 A 3-ph. BSCCO 2212 bulk Nexans Germany / 2008 Resistive-inductive 63.5 kv, 1.8 ka 1-ph. BSCCO 2212 bulk KEPRI Korea / 2007 Resistive 13.2 kv, 630 A 3-ph. BSCCO 2212 bulk General Atomics US / 2002 Diode bridge 7.2 kv, 1.2 ka 1-ph. BSCCO 2223 tape Yonsei University Korea / 2004 Diode bridge 3.8 kv, 200 A 3-ph. BSCCO 2223 tape CAS China / 2005 Diode bridge 6 kv, 1.5 ka 3-ph. BSCCO 2223 tape Innopower China / 2007 DC biased iron core 20 kv, 1.6 ka 3-ph BSCCO 2223 tape Zenergy power Aus,US,D / 2009 DC biased iron core - 3-ph BSCCO 2223 tape KEPRI Korea / 2004 Resistive 3.8 kv, 200 A 3-ph. YBCO thin films CRIEPI Japan / 2004 Resistive 1 kv, 40 A 1-ph. YBCO thin films Mitsubishi Japan / 2004 Resistive 200 V, 1 ka 1-ph. YBCO thin films Siemens / AMSC Germany / 2007 Resistive 7.5 V, 267 A 1-ph. YBCO coated cond. AMSC / Siemens US / Germany / - Resistive 66 kv, - 3-ph. YBCO coated cond. Hyundai / AMSC Korea / 2007 Resistive 13.2 kv, 630 A 1-ph. YBCO coated cond. IGC Superpower US / 2009 Resistive 80 kv, - 3-ph. YBCO coated cond. Rolls Royce UK / - Resistive 6.6 kv, 400 A - MgB 2 EPRI / Powell US / 2004 Power electronics 8 kv, 1.2 ka 3-ph. Without SC Siemens Germany / 2004 Power electronics 6.9 kv, 25 MVA 3-ph. Without SC 1) year of test 2) phase to ground voltage 534